A conventional polarized electromagnet device, as shown in FIG. 1, is of four-magnetic-gap type. In FIG. 1, reference numeral 1 designates two outer yokes which are substantially U-shaped. Each outer yoke 1 is made up of a central piece 1a and leg pieces 1b extending from the central piece 1a on both sides. The central portion of each central piece 1a is in contact with the magnetic pole surface of the north pole of a permanent magnet 2, and the magnetic pole surface of the south pole of each permanent magnet 2 is in turn in contact with the central piece 3a of an inner yoke 3, to form a stationary side. The inner yokes 3 are L shaped, and made up of a central piece 3a and a leg piece 3b.
A magnetic coil 4 is provided inside the inner yokes 3, and is penetrated by a plunger or movable magnetic pole bar 5, as shown in FIG. 1. The ends of the plunger 5 are fixedly secured to the central portions of armatures 6 and 7. Non-magnetic plates 8 are provided on the outer surfaces of the armatures 6 and 7. The plunger, the armatures 6 and 7, and the non-magnetic plates 8 form a movable block which is movable between the legs 1b of the outer yoke 1.
FIG. 1 illustrates the configuration of a conventional device in its "return state" when no voltage is applied to the electromagnetic coil 4. In this state the armature 7 and corresponding non-magnetic plate 8 is confronted with the legs 1b of the outer yokes 1. Gaps A, B, and C are formed between the armature 6 and the legs 1b of the outer yokes 1, between the armature 6 and the legs 3b of the inner yokes 3, and between the armature 7 and the end of the central pieces 3a of the inner yokes 3, respectively.
When a sufficient voltage is applied to the electromagnetic coil 4 to drive the movable block to an "attraction state" such that armature 6 and corresponding non magnetic plate 8 is confronted with the legs 1b on the opposite side of the outer yoke 1, gaps B, C, and D are formed. Gap D corresponds to that spacing created between the armature 7 and the legs of the outer yokes 1.
The solid line arrows in FIG. 1 represent the direction of the magnetic flux of one of the permanent magnets 2. A similar flux pattern exists for the other permanent magnet 2, but is not shown. The X-Y arrow represents the directions in which the movable block may be driven.
FIG. 2 graphically shows the magnetic attraction forces and spring load charcteristics of the conventional polarized electromagnetic device of FIG. 1. In FIG. 2, the horizontal axis represents the stroke of the movable block which corresponds substantially to the length of the magnetic gap A, and the vertical axis represents magnetic attraction forces and spring loads.
The composite attraction force P'.sub.1 shown in FIG. 2 with a solid line is the vector composition of the magnetic flux of the permanent magnets 2 and that of the electromagnetic coil 4, acting in the X direction. This composite attraction force P'.sub.1 is provided when a rated voltage is applied to the electromagnetic coil 4. Similarly, a composite attraction force P'.sub.2 is provided when a minimum allowable voltage (70% of the rated voltage in this example) is applied to the electromagnetic coil 4.
The reference character P'm in FIG. 2 designates the attraction force of permanent magnets 2. The fact that the attraction force of permanent magnets 2 act in the negative direction on the return side indicates that the movable block is urged in the Y direction.
In the conventional device, a spring load P'.sub.3 of a return spring (not shown) is applied to the movable block of FIG. 1 in the Y direction at all times. In addition to the spring load P'.sub.3, a spring load P'.sub.4 provided by a main contact spring and an auxiliary contact spring (both not shown) is also applied to this movable block from the time when the movable block reaches a predetermined position while moving in the X direction.
Spring load P'.sub.3 is shown graphically by a broken line in FIG. 2. The cumulative effect of these forces is represented by a composite spring load P'.sub.5 on the attraction side. Spring load P'.sub.5 is shown in FIG. 2 by a one-dot chain line.
In order that the conventional polarized electromagnet device will operate effectively as an electromagnetic contactor, the composite spring load P'.sub.5 must be lower than the composite attraction force P'.sub.2. The attraction force P'm of the permanent magnets acts greatly in the negative side, i.e., in the Y direction on the return side. In the device shown in FIG. 1, however, the composite attraction force P'.sub.2 is smaller than the spring load P'.sub.3 of the return spring, and is insufficient to urge the movable block towards the attraction side. Therefore, in the conventional polarized electromagnet device, a spring load P'.sub.X must be added in the operating X direction so that the composite spring load P'.sub.5, as indicated by the one dot chain line in FIG. 2, will become lower than the composite attraction force P'.sub.2.
As is apparent from the above description, the conventional polarized electromagnet device is disadvantageous in that, when used as an electromagnetic contactor, its spring load characteristic is intricate and its construction is unnecessarily complicated. Another disadvantage of the conventional device is that the contact pressure is low. As can be seen from FIG. 2, the attraction force P'm of permanent magnets 2 acts greatly on the negative side, and therefore the composite attraction force P'.sub.2 or P'.sub.1 obtained by adding the attraction force of the electromagnetic coil 4 thereto is small.